Heretofore, liquid crystal compounds are important as compounds for use in the manufacture of displays of various electrooptic devices. Among the various liquid crystal compounds, compounds having an alkenyl group as a side chain are suitable as the liquid crystal compounds for STN, and the various compounds having the alkenyl group have been synthesized and evaluated, and some of these compounds have been put to practical use. For example, the following compounds (s-1) to (s-3) have been disclosed in Mol. Cryst. Liq. Cryst., 122, p. 241 (1985) and Japanese Patent Application Laid-open No. 83136/1986, the following compounds (s-4) and (s-5) have been revealed in Japanese Patent Application No. 92740/1994, and the following compounds (s-6) and (s-7) have been disclosed in DE19520246Al. ##STR2##
However, the above liquid crystal compounds are poor in steep threshold properties which are required for the liquid crystal compounds for STN, and all of them have strong smectogenic properties, so that if a liquid crystal composition containing a large amount of such a compound is prepared, there is a problem that the compatibility of such a liquid crystal compound with another liquid crystal compound in the liquid crystal composition at a low temperature is low. In particular, the compounds (s-4) to (s-7) have each a high viscosity, and so there is a large problem that a response speed of the liquid crystal composition using such a compound is low.
Compounds (s-8) and (s-9) in which a fluorine atom is directly bonded to a double bond have been disclosed in Japanese Patent National Publication (Kohyo) No. 500343/1994, and a compound (s-10) has been revealed in Japanese Patent National Publication (Kohyo) No. 502627/1992. However, the compounds (s-8) and (s-9) are extremely poor in chemical stability (particularly, heat stability), and therefore the use of these compounds is practically impossible. Moreover, in Japanese Patent National Publication (Kohyo) No. 502627/1992, there have been disclosed compounds alone in which a terminal group is limited to a polar group such as a halogen atom.